1. Technical Field of the Invention
The present invention relates to a semiconductor memory device, and particularly to a semiconductor memory device utilizing phase change material.
2. Description of the Related Art
Memory devices have been in existence for decades to store data or other information. There are presently many different types of both volatile and nonvolatile memories. One type of nonvolatile memory is a phase change memory. Phase change memories utilize phase change materials. Phase change materials may exhibit at least two different states. The states may be called the amorphous and crystalline states. Transitions between these states may be selectively initiated. The states may be distinguished because the amorphous state generally exhibits higher resistivity than the crystalline state. The amorphous state involves a more disordered molecular or atomic structure. Generally, any phase change material may be utilized. In some embodiments, however, thin-film chalcogenide alloy materials may be particularly suitable.
The phase change of phase change material may be reversible. Therefore, phase change material in a phase change memory may change from the amorphous to the crystalline state and may revert back to the amorphous state thereafter, or vice versa, in response to certain changes.
In effect, each memory cell may be thought of as a programmable resistor, which reversibly changes between higher and lower resistance states.
In some embodiments, the phase change memory cell may be configured in any of a relatively large number of states. That is, because each state may be distinguished by a distinct resistance or resistance range, a number of resistance-determined states may be possible, thereby allowing for the storage of multiple bits of data in a single phase change memory cell.
A variety of phase change alloys are known. Generally, chalcogenide alloys contain one or more elements from Column VI of the periodic table.
Phase change materials in phase change memories may be programmed from a relatively low resistance (crystalline) state to a relatively high resistance (amorphous) state by applying to the phase change material being programmed a pulse of current having a voltage level greater than a switching threshold voltage of the phase change material. Programming the phase change materials from the relatively high resistance state to the relatively low resistance state is performed by applying to the phase change material a pulse of current of a lesser current level, but also at a voltage greater than the switching threshold of the phase change material.
In the semiconductor industry, there is an ongoing demand for integrated circuits that are smaller, faster and less expensive to manufacture. Based upon the foregoing, there is a need for memories, such as phase change memories, having improved performance.